Sets the default send options applied to messages submitted over SMTP with this API key — which IP pool to send from, the content category, tags, and open/click tracking. A field you omit is left unchanged. If no config exists yet for this key, it's created first with the documented defaults (your organization's default pool, transactional category, no tags, tracking on), and omitted fields take those defaults. ID of the IP pool to send from (`ipp_` prefix), or `ipp_shared` to route through the shared pool explicitly. Send `null` to reset to your organization's default pool, or omit to leave unchanged. An unknown pool, or a pool with no dedicated IPs available to send from, is rejected with a `422`. Content classification — independent of which endpoint messages are submitted through. Controls suppression policy: `marketing` blocks on all suppression reasons; `transactional` allows delivery through complaint and unsubscribe suppressions. Omit to leave unchanged. Structured `{name, value}` labels applied to every message submitted over SMTP with this key. Send an empty array to clear all tags, or omit to leave unchanged. Tag name. ASCII letters, digits, underscore, and hyphen only. Case-sensitive. Maximum 32 characters. Tag value. ASCII letters, digits, underscore, and hyphen only. Case-sensitive. Maximum 64 characters. Whether to track open events for messages submitted over SMTP with this key. Omit to leave unchanged. Whether to track click events for messages submitted over SMTP with this key. Omit to leave unchanged. ID of the IP pool that SMTP sends with this key use, or `ipp_shared` for the shared pool. `null` when this key uses your organization's default pool. Content classification applied to messages submitted over SMTP with this key. Controls suppression policy: `marketing` blocks on all suppression reasons; `transactional` allows delivery through complaint and unsubscribe suppressions. Structured `{name, value}` labels applied to every message submitted over SMTP with this key — the same tags used by the email sending API. See EmailMessageSendRequest for how tags are used for filtering and analytics. Tag name. ASCII letters, digits, underscore, and hyphen only. Case-sensitive. Maximum 32 characters. Tag value. ASCII letters, digits, underscore, and hyphen only. Case-sensitive. Maximum 64 characters. Whether open events are tracked for messages submitted over SMTP with this key. Whether click events are tracked for messages submitted over SMTP with this key.